1. Field of the Invention
This invention relates generally to separation of the gases in air and more particularly relates to an apparatus and method for extracting oxygen from ambient air to provide an inexpensive, portable, personal, oxygen source.
2. Description of the Related Art
Many individuals require or benefit from an oxygen rich source to improve their respiration. Such oxygen is conventionally supplied to a patient from a high pressure tank of compressed oxygen. The oxygen is filled into the tank by a large scale commercial operation that separates oxygen from air, compresses the oxygen and fills the tanks. The tanks are then distributed through a distribution system to individual patients. When the tanks are emptied, they are returned and refilled.
Prior art methods of oxygen separation include the use of a turboexpander and liquefaction by contacting the ambient air against a sufficiently cold surface that one or more component gases in the air are condensed. Often the components of air are liquefied and separated for sale as individual gases, although liquid nitrogen has also been used to condense the oxygen in air. Liquefaction in the prior art typically expends significant cooling energy to accomplish the liquefaction. After accomplishing the liquefaction, the remaining energy is put to no further use but is lost because it is carried away in the separated components and/or lost in a compression operation.
Although conventional oxygen separation and distribution systems have been of great benefit to patients who need oxygen, it has several disadvantages which add to the cost of the oxygen and make transporting the oxygen source difficult. Compression of the incoming air to at least 1 bar above atmospheric pressure is needed by prior art systems to accomplish conventional separation and movement of fluids through the system. The recovered oxygen also must be highly compressed so that the total mass of oxygen contained in the tanks is large enough to apportion the transportation cost for the heavy tanks over a larger mass of oxygen and therefore reduce the per unit cost of transportation and distribution.
Compression is typically accomplished at one or both of two stages of the separation process. Incoming air is compressed prior to separation not only to generate a pressure differential across the separator which is necessary to propel the air and its separated components through the separation system but also to provide adequate separation efficiency or rate of production. Furthermore, after separation, the separated components are often further compressed for filling into tanks. However, compressors are not only noisy and of significant weight, but also they are costly and consume significant energy, especially when designed to provide uncontaminated gases suitable for human respiration, and therefore add to the energy cost for producing oxygen. That energy is then lost when the oxygen is returned to substantially atmospheric pressure so it can be administered to a patient. Additionally, the need to transport heavy tanks adds a transportation cost to the oxygen in addition to the inconvenience of handling the tanks for suppliers, the patients and any care givers.
This conventional oxygen supply system has continued for decades because the separation and compression of oxygen in accordance with prior art techniques could be more efficiently and therefore less expensively accomplished by large scale, mass production systems devoted to the separation of the components of air for resale as compressed or liquid single components at various purity levels. The scaling down of the conventional separation systems for individual home use is impractical and would be prohibitively expensive.
For example, U.S. Pat. No. 5,893,275 describes a system as intended for home use. However, it requires a multiplicity of stages including a compressor, a first stage separator using an adsorptive process, a membrane separator or an ionic conduction system, and a liquefier which liquefies but does not separate the gases by liquefaction. U.S. Pat. No. 5,704,227 illustrates the use of a liquid nitrogen coolant as a cooling medium for condensing a volatile compound, such as a lower aldehyde, a glycol compound and water, from a gas such as nitrogen. Although a heat exchanger is used to pre-cool incoming gas, this system requires a liquid nitrogen source which makes such a system impractical for home use.
There is, therefore, a need for an oxygen separation system that can be economically implemented on a small scale so it will be practical for home use and is sufficiently small in size and weight that it can be made portable.
It is an object and feature of the present invention to provide an oxygen separation system which requires no compression and therefore can operate more energy efficiently by eliminating both the need to compress the incoming air prior to separation and by eliminating the need for compression of the oxygen for storage in tanks.
Another object and feature of the invention is to provide an oxygen separation system which separates the oxygen from air by direct liquefaction of only the oxygen.
Another object and feature of the invention is to provide an oxygen separation system using liquefaction but which recovers the cooling energy by using it in the liquefaction process and, as a result of recovering and using the energy, reduces the energy costs and permits use of simpler components which require less energy input.